Yep! "An apple" is any apple in the world, and "the apple" is a specific apple-- either there is only one apple, or it is the one being talked about. Usually you have to introduce with a/an before using the: "A ball bounced down. Then the ball bounced away." But if it's very obvious, you don't always have to: "I was having Doritos, and I ate everything in the bag." ("The bag" is clearly a Dorito bag and needn't be declared.) If you forget to declare, people will be confused: "The dog peed on my fence again." ("What dog? Were we talking about a dog?") But if you declare it more than once, it sounds like there are more than one object: "A van ran a red light, and then a van followed me home." ("There must be a lot of strange vans today.") Protip: you can use "it" in cases where you refer to the subject multiple times without adding more in between, skipping the "the" part altogether: "A purple, flying elephant did a dance, and then the purple, flying elephant flew away." vs. "A purple, flying elephant did a dance, and then it flew away." Both are correct, but one is significantly shorter! But be careful: "A robot punched a tree, and then it fell over." ("Which thing fell over? The robot or the tree?") So use "the" when there's more than one thing. Have fun!